The invention relates to metal/air batteries, and particularly such batteries having multiple cells.
Metal/air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having opposite surfaces respectively exposed to the atmosphere and to the aqueous electrolyte of the cell, in which (during cell operation) oxygen dissociates while metal of the anode oxidizes, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuitry can be connected. Present-day commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) containing a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight, and ability to function as anodes in metal/air batteries using neutral electrolytes such as sea water or other aqueous saline solutions.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface. The discharge reaction for this cell may be written EQU 4Al+3O.sub.2 +6H.sub.2 O.fwdarw.4Al(OH).sub.3.
As the reaction proceeds, large amounts of the aluminum hydroxide reaction product forms in the space between anode and cathode and this ultimately interferes with cell operation, necessitating periodic cleaning and electrolyte replacement. It will be appreciated that cleaning and electrolyte replacement become quite complicated when the battery has multiple cells.
The provision of a metal/air battery for emergency situations is proposed in Watakabe, "Magnesium-Air Sea Water Primary Batteries", Solar Cells, Vol. II (Cleveland: JEC Press Inc., 1979). This publication shows a "life-torch" with a series-connected twin cell battery of "inside-out" construction, namely a pair of spaced-apart magnesium anodes having a pair of cathodes interposed between them and mutually defining a common air space. Each anode-cathode pair is surrounded by a separate electrolyte space (within a housing) to prevent or minimize electrolytic shunting between the battery cells. As those skilled in the art can appreciate, since the anodes of a pair of series-connected metal-air battery cells are at different potentials, the existence of a current path through the electrolyte between the anodes of the respective cells will cause undesired shunting of current and can significantly impair cell efficiency.
Utilization of a battery constructed in accordance with the above-cited publication would require pouring saline electrolyte into each of the battery inlets. As one can appreciate, the pouring of electrolyte into separate inlet ports can be extremely difficult, especially in the dark. An easier method of filling electrolyte into the batteries is desirable for land applications. Moreover, the device of the above-cited publication is evidently designed for a single use in a marine emergency; for a routine consumer land application, it would be desirable to have a battery that could be repeatedly activated by pouring electrolyte into the cells, and repeatedly de-activated by removing the electrolyte from the cells and cleaning out reaction products formed within the cells, without the hindrance of separate tanks for the two cells.
Also, it would be desirable to retard the accumulation of reaction product in the anode-cathode gap of a metal/air cell or battery, such as an aluminum/air battery, thereby to prolong the period of active use of the cell or battery between cleanings. In this regard, it has heretofore been proposed to provide a relatively wide anode-cathode gap for providing flow of fresh electrolyte around the gap edges, generally parallel to the electrode surfaces; but cell efficiency decreases with increasing anode-cathode distances. Another proposal, set forth in the Handbook of Batteries and Fuel Cells (McGraw-Hill, 1984), p. 30-11, is to prevent hydroxide gel formation by employing a caustic electrolyte, but caustic electrolytes are disadvantageous (as compared to saline electrolyte) from the standpoint of convenience, cost, and safety in handling. Thus, it is highly desirable to have a battery capable of functioning with saline electrolyte where the use of caustic may not be desired.
It is an object of the present invention to provide a multi-cell metal/air battery which is compact, easy to operate, easy to clean and re-use and having excellent performance characteristics.